An intruder aircraft avoidance maneuver is a tricky maneuver, since the crew is required to avoid the trajectory of the intruder aircraft while remaining in control of its own aircraft and of the trajectory of the latter. Two problems may in particular occur during such a maneuver:                the pilot pushes the aircraft to the limit or outside of its flight envelope. This triggers other alarms which get added to the initial alarm;        the pilot quits his flight plan to carry out the avoidance. In this case he risks crossing the trajectory of a third aircraft. This often results in a disruption of the air traffic, in particular in the approach zones to large airports.        
It is known that an anticollision system, in particular of TCAS type (Traffic alert and Collision Avoidance System), makes it possible to monitor the trajectories of the aircraft in proximity to the aircraft considered and to represent their respective positions on a viewing screen, for example of ND (Navigation Display) type.
This anticollision system is based on an exchange of information by way of transponders. With the aid of the altitude and of the distance, which are exchanged for example every second, said anticollision system calculates the trajectory of any intruder aircraft. It then estimates the potential danger and calculates an appropriate maneuver to avoid it. This maneuver is executed solely in the vertical plane.
Intruder aircraft are generally classed into several categories according to their proximity. Thus the following alerts or alarms are distinguished:                a traffic advisory which makes it possible to signal the machines which are between 25 and 40 seconds from the aircraft. The pilot must monitor the evolution of the trajectories of these machines, but no maneuver or limitation is imposed upon him; and        a firm alarm or alert (referred to as an alarm hereinbelow) [resolution advisory] which forewarns of close danger (less than 25 seconds). On the basis of the data relating to the two aircraft (altitude, distance and speed), the anticollision system devises two possible maneuvers:                    a first maneuver associated with a preventive alert, which consists in maintaining the current trajectory;            a second maneuver associated with a corrective alert, which consists in executing a climb or a descent at a rate defined by the anticollision system until the danger is cleared. This maneuver is performed solely in the vertical plane.                        
During a firm alarm or alert of resolution advisory type, a particular signpost is generally presented on a vertical speed scale of the primary piloting screen of the aircraft. Two zones are displayed on this scale:                a red zone which represents a prohibited vertical speed zone; and        a green zone in which the pilot must place the vertical speed of the aircraft in order to avoid the intruder aircraft.        
In case of corrective alarm, the pilot is required to disengage the automatic pilot, as appropriate, and to perform the avoidance maneuver manually. To do this he must actuate the control stick so as to place the vertical speed in the aforesaid green safety zone. In practice, pilots are required to track the limit vertical speed between the red zone and the green zone.
However, experience shows that the tracking of a vertical speed preset is not intuitive for a pilot. Specifically, the vertical speed is not a primary piloting parameter, like the attitude or the air speed for example. Pilots thus tend to exceed the preset, which may bring about:                a strong variation in the load factor, which is detrimental to the comfort and to the safety of the passengers;        an abrupt variation in the speed and in the angle of incidence, which involves a risk of exiting the flight envelope; and        a significant deviation of the trajectory with respect to the initial trajectory, which disrupts the air traffic in zones of dense traffic.        
To attempt to remedy these drawbacks, a known solution advocates displaying on the primary piloting screen an avoidance preset expressed in terms of attitude. To do this, the vertical speed preset is converted into a value of attitude, which is easier to control by the pilot. This representation is known by the name “pitch cues”.
However, the manual avoidance implemented in this case remains very dynamic and does not cope with all the problems previously alluded to (in particular because the pitch or attitude indications are calculated with a relatively high gain so as to induce the pilot to carry out a fast avoidance maneuver.